On 1 July 2010 16:04, Christopher Tauss
Hello -
I just a day or so ago downloaded Hakell and am playing around with it, and I came upon this problem with WinGHCi:
I am able to enter a multi-line "do" statement that works if I use brackets and semi-colon like so:
Prelude> :{ Prelude| let main2 = do { Prelude| putStrLn "Please enter your name: "; Prelude| name <- getLine; Prelude| putStrLn ("Hello, " ++ name ++ ", how are you?") } Prelude| :} Prelude> main2 Please enter your name: CT Hello, CT, how are you? Prelude>
Note there is no indentation. This makes sense to me because the :{ :} just take all the lines in between and make it one line.
But it seems to me to be IMPOSSIBLE to input this into WinGHCi using indentation like most of the code samples seem to do.
Should I just be satisfied that it works using brackets/ semi-colons? Or is there something obvious that I am missing that allows for indentation?
Typically, ghci (including WinGHCI) and Hugs are used to evaluate and experiment with code, rather than writing it. Write your actual code in a file and load it with :load (or :l for short) into ghci. In this sense, they aren't "real" REPLs in that you can't define new data types, classes, etc. in them (you can write functions with a let statement, but it gets cumbersome for long functions). It might be better off thinking of the prompt as being individual lines in a big do-block for a specialised version of the IO monad (in the sense that normally just entering "5+4" wouldn't typecheck let alone print the result, etc.).
Thanks in advance. It's issues lik this that keep me up into the depths of night.
Hopefully you can now get to sleep ;-) -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com